Peter Hosey explains what warnings he uses and why. It’s good, but long. Fortunately, you can just grab a script, and enable those warnings in your Xcode projects.
Warnings = Errors
If I could force just one compiler flag on everyone who’s code I use, it would be TREAT_WARNINGS_AS_ERRORS. As a rule, things don’t get improved if they aren’t broken. (How many times have you said “I’ll come back and fix this code later”? Yeah.) Warnings fester and grow on each other, until they cause a real breakage. It’s an inescapable evil of building software with finite resources.
If a warning isn’t worth stopping the build over — it’s not worth checking for in the first place.
Use the Latest Tools
Specifically, if you aren’t using Snow Leopard and Xcode 3.2 to build your Objective-C code, you are crazy. Trust me, painless static analysis is worth upgrading for. It catches maddening memory leaks, not just trivial type errors, like adding an int to an NSArray, that you would catch immediately.